1. Field of the Invention
The present invention relates to a high strength hot rolled steel sheet for wheel rims, and more particularly to a high strength hot rolled steel sheet for wheel rims which is excellent in DC weldability and formability.
2. Discussion of the Background
As a measure to reduce the weight of automobile bodies for saving fuel costs, there has been an attempt to modify the material for the automobile bodies by employment of high strength hot rolled steels, coupled with an attempt to reduce the size of automobile bodies. In particular, a reduction in the weight of wheels has been deemed effective in curtailing the fuel costs, and extensive studies have been made on the application of high strength steel sheets to wheel rims.
Wheel rims have hitherto been produced by cutting a steel sheet to predetermined width and length, forming the cut sheet piece into a ring form and welding the ends thereof to each other, followed by several steps of roll forming. For the welding, the technique of flash butt welding has hitherto been employed; but, in recent years, there is a trend toward the adoption of DC butt welding for this purpose in view of its merits that the welding environment is not worsened by dusts or noise, because of the absence of flashing, and about 1% increase in the yield can be expected because no part of the material is consumed in flashing.
However, the use of DC butt welding encounters a new problem in the production of the wheel rims. Namely, a situation which has mattered little in the conventional use of steel sheets with a tensile strength of not higher than 45 kgf/mm.sup.2 comes to be a problem in the case of using high strength steel sheets with a higher C or Mn content. The problem, is that due to the large-current welding or the welding condition wherein an electric current is passed during the upset process or even after the upset process, austenite grains in the steel are coarsened by the time of completion of welding, which lead to high hardenability in the subsequent cooling process, and accordingly, the austenite is likely to be transformed into martensite or hard bainite, resulting in an extremely high hardness of the weld zone. Consequently, cracking tends to occur starting from the welded interface during the subsequent working. Although the limit of the maximum hardness of the weld zone has not hitherto been made clear, it has been found by the present inventors' studies that the upper limit of the maximum hardness is preferably about 300. However, in most cases where conventional high tensile steels are welded by DC butt welding, the maximum hardness (Hv) of the weld zone exceeds 300, which is higher than those of flash butt welded steels by at least 100, and the structure of the DC butt welded steel is nearly martensite. Furthermore, brittle rupture surfaces are present in the weld zone, and the Charpy absorption energy (vE.sub.0) at 0.degree. C. is low.